1. Field of the Invention
The present invention relates to an optical communication system, in particular, to an optical network with fault recovery capabilities.
2. Discussion of the Background
The explosive growth of the Internet, which has millions of users, has and will continue to generate enormous data traffic over the backbone networks of the Internet. These backbone networks correspondingly require greater bandwidth in the transmission paths to support the many user applications, from email to streaming video. By the same token, any disruption along the transmission paths of the backbone networks would adversely affect countless numbers of users. Such a disruption may stem from a cable cut, or some other fault (e.g., equipment failure).
The effects of the disruption range from increased response times to data loss. When the traffic is mission critical, any loss of information is unacceptable. Hence, along with greater bandwidth demand from the users, service providers are required to guarantee extremely high network reliability, which in turn requires spare capacity.
It has been recognized that fiber optic cables hold the promise of being able to rapidly transport vast amounts of traffic. Thus, development in optical communications networks has steadily progressed. SONET (synchronous optical network), which is promulgated by the American National Standards Institute (ANSI), has emerged as an accepted standard defining the transport and management of data traffic over fiber optic transmission systems. An important feature of SONET is its ability to automatically recover from network faults.
A conventional SONET ring network, as schematically shown in FIG. 16, utilizes four optical fibers to transport data, in which two optical fibers 1a and 1b are designated as working, while the other two 2a and 2b are considered protection fibers. The fibers 1 and 2 connect five SONET nodes, N1-N5. The optical signals within the working fiber pairs 1a and 1b are normally transmitted in opposite directions, as indicated by the arrowheads. Similarly, within the protection fiber pairs 2a and 2b, the fiber line 2a carry optical signals in an opposite direction to the optical signals that are transmitted by fiber line 2b. As will be discussed more fully below, the optical signals that are transmitted via the working fiber pairs 1 are switched over to the protection pairs 2a and 2b upon occurrence of a fault; SONET nodes N1-N5 are SONET add/drop equipment for adding and dropping the optical signals to and from the working optical fibers 1a and 1b, and the protection optical fibers 2a and 2b. 
The add/drop function within SONET is performed at the electrical level; accordingly, a SONET node must temporarily convert the optical signals into electrical signals before processing the optical signals. Based upon examination of the electrical signals, the SONET node can determine whether a fault has occurred in the network.
FIG. 16 shows an operational scenario in which all the transmission paths 1a, 1b, 2a, and 2b as well as the nodes N1-N5 are fully functioning. In this example, when an optical signal is added to the node N3 (as indicated by the arrowhead A) and is destined to node N1. The optical signal follows the shortest route, and thus, is transmitted in a clockwise direction via optical fiber 1b to the destination node N1. The optical signal is then dropped at node N1, as indicated by the arrowhead D. On the other hand, if an optical signal is added to node N1, as indicated by the arrowhead A, the optical signal is transmitted counterclockwise via optical fiber 1a to the destination node N3 (which is the shortest route). Next, the transmitted optical signal is dropped at node N3.
Although not illustrated in FIG. 16, a node connected to another ring network is provided with cross-connect functionality to switch the optical signals over to the other network. That is, under the conventional approach, to link one optical communications network to another, an electrical cross-connect converts the optical signals into electrical signals, then reconverting some of the electrical signals, if necessary, into optical signals for transmission over the other ring network.
FIGS. 17A and 17B illustrate the operation of the SONET ring, whereby the optical fibers between nodes N1 and N2 are totally and partially down, respectively. The protection function of the SONET layer works to set alternative transmission paths in the ring network. Under these fault conditions, alternative routing of the optical signals can be achieved using two protection schemes: (1) ring protection, and (2) span protection. The architecture of the SONET ring in FIGS. 17A and 17B resembles that of FIG. 16, in which five nodes are interconnected via four optical fibers (i.e., an working pair 1 and a protection pair 2).
FIG. 17A shows an alternative routing scheme known as xe2x80x9cring protectionxe2x80x9d, in which all four optical fibers (i.e., working optical fibers 1a and 1b and protection optical fibers 2a and 2b) simultaneously fail. Under this scenario, a fault Os occurs between the node N1 and the node N2 in the ring network. The ring protection system sets an alternative route by detecting faults in the fiber links, in this case, between the nodes N1 and N2. Based upon detection of fault Os, the ring protection system connects the working optical fiber 1a to the protection optical fiber 2b, and the working optical fiber 1b to the protection optical fiber 2a. According to the ring protection scheme, the optical signal added in the node N3, as indicated by the arrowhead A, is carried by the working optical fiber 1b. The shortest route to the destination node N1 is in the clockwise direction via node N2. Upon the optical signals reaching node N2, node N2 recognizes that both the working pairs 1 and the protection pairs 2 are disconnected due to some fault. Accordingly, node N2 switches the optical signal over to the protection optical fiber 2a. The optical signal, thereafter, is transmitted in the counterclockwise direction to the node N1, where the optical signal is dropped (as indicated by the arrowhead D) from the protection optical fiber 2a. 
In a converse situation, node N1 adds an optical signal to the protection optical fiber 2b. After the optical signal is transmitted clockwise, it temporarily passes through the destination node N3 so that node N2 may switch the optical signal over to the working optical fiber 1a. Node N2 transmits the optical signal in a counterclockwise direction and drops the optical signal from the working optical fiber 1a at destination node N3, as indicated by the arrowhead D.
FIG. 17B shows the operation of an alternative routing scheme known as xe2x80x9cspan protectionxe2x80x9d. As shown, a fault Os occurs between the node N1 and the node N2 in the ring network, in which the working optical fibers 1a and 1b experience failure. Under this scenario, the span protection system detects a fault between nodes N1 and N2, involving both working optical fibers 1a and 1b. After detection of this fault Os, the span protection system utilizes protection optical fibers 2a and 2b in that particular span where working lines 1a and 1b failed.
In the example of FIG. 17B, node N3 adds an optical signal to the working optical fiber 1b, and transmits the optical signal clockwise to node N2. Node N2 then switches the optical signal over to protection optical fiber 2b, transmitting the optical signal in a clockwise direction, and dropping it in the destination node N1.
In the case where an optical signal is added at node N1 to the protection optical fiber 2a, node N1 transmits the optical signal in a counterclockwise direction to node N2. Node N2 switches the optical signal over to the working optical fiber 1a, and transmits the optical signal counterclockwise, dropping the optical signal at node N3.
Although SONET does provide the necessary fault recovery mechanisms, it has some limitations with respect to bandwidth. Because SONET is based on a time division multiplexing (TDM) scheme, the achievable transmission rates are constrained by physical characteristics of the fiber optic cables. In some instances, to obtain more bandwidth, more fiber optic cables need to be added to the install base, which in a number of cases is a cost prohibitive proposition.
Therefore, service providers have turned to wavelength division multiplexing (WDM) to meet the need for higher bandwidths. WDM technology has emerged to address the bandwidth demand by increasing the number of communication channels in an optical network; notably dense wavelength division multiplexing (DWDM) has opened a new door to very high bandwidth. DWDM systems utilize different, closely-spaced wavelengths to carry information.
The SONET ring network of FIG. 16 can be converted to a WDM network by transmitting optical signals that are wavelength-multiplexed signals; this requires that the appropriate WDM optical add/drop equipment be used for nodes N1-N5.
However, as evident from the above discussion, the SONET add/drop function and associated protection function are attained at the electrical level. That is, these functions require that the optical signals be converted to electrical signals for processing and then converted back to optical signals for transport. This process of conversion and reconversion is also characteristic of general WDM communications systems. Furthermore, the WDM optical signals not only increase the cost of SONET equipment, but increases the complexity of the node function. Accordingly, this process of converting from optical to electrical and vice versa is inefficient.
It is therefore desirable to obtain SONET layer protection functions at the optical level. To achieve this objective, the optical add/drop function, and the optical cross-connect function need to be carried out without optical to electrical conversion.
One conventional approach to providing a protection function in an optical layer is to deploy a waveguide type 4xc3x974 matrix optical switch, which is made up of 16-pieces of 1xc3x972 type and 2xc3x972 type unit optical switches in an optical network of FIG. 16. The unit optical switches in this conventional matrix optical switch are provided as planar waveguides on the same substrate. A drawback with the waveguide type 4xc3x974 matrix optical switch approach is the use of a large number of the unit optical switches (i.e.,16). As a consequence, the corresponding insertion loss can reach up to 6.6 dB. Hence, this system requires an expensive optical amplifier to compensate for the insertion loss of this 4xc3x974 matrix optical switch. Additionally, the waveguide type 4xc3x974 matrix optical switch provides a large number of connection states between the I/O ports; e.g., the number of states can be 24 (4xc3x973xc3x972xc3x971=24).
However, the protection function, as described above, can be achieved with less than 24 connection states. Accordingly, this arrangement provides a less than efficient and more costly solution. Another drawback with the waveguide type 4xc3x974 matrix optical switch concerns electrical power consumption. The matrix optical switch requires a continuous voltage to all four optical switches in order to keep the desired connection state between the I/O ports.
In the optical line switching system described above, when the protection function is invoked, the plurality of unit optical switches are required to be simultaneously switched. Hence, the probability of malfunction increases, which negatively impacts reliability of the system.
Based on the foregoing, there is a clear need for improved approaches for providing protection function in an optical layer.
There is a need to minimize the cost of network operation by reducing the number of optical switch units and optical amplifiers.
There is also a need to minimize electrical power consumption in an optical communications network.
There is further a need for improving network reliability of an optical communications network.
Based on the need to supply reliable network services in a high bandwidth environment, an approach for performing fault recovery at the optical layer is highly desirable.
According to an aspect of the invention, an optical line switching system comprises an optical switch connected to working optical fibers and protection optical fibers. The optical switch is configured to switch over optical signals among the working optical fibers and the protection optical fibers. The optical switch comprises a plurality of unit optical switches and a common driving mechanism that is configured to perform simultaneously switching operation of the unit optical switches to alter the switching state of the optical switch. A plurality of monitoring devices are coupled to the working optical fibers and the protection optical fibers and configured to monitor the optical signals transmitted over the working optical fibers and the protection optical fibers. The monitoring devices are configured to output selectively monitoring signals that indicate one or more faults in the working optical fibers and the protection optical fibers. A control device is coupled to the monitoring devices and configured to output a control signal to the optical switches to effect an optical protection scheme by selectively changing switching states of the optical switches in response to the monitoring signals. Under this approach, the number of optical switches is minimized, while retaining the optical protection functions of the ring and span protection schemes.
According to one aspect of the invention, a method of providing an optical protection system that utilizes an working transmission path and a protection transmission path comprises transmitting optical signals over the working transmission path and the protection transmission path. The method includes performing an optical add/drop function and a multiplex/demultiplex function on the optical signals and monitoring the transmission of the optical signals over the transmission paths. A monitoring signal is generated to indicate whether a fault in the transmission paths has occurred. Further, the method includes outputting control signals to a plurality of optical switches to effect an optical protection scheme in response to the monitoring signal. The method also includes altering switching states of the plurality of optical switches in response to the control signal of the outputting step. Each of the plurality of optical switches comprises unit optical switches that are simultaneously operated by a common driving mechanism. The above arrangement advantageously reduces power consumption.
In yet another aspect of the invention, an optical communications network for providing fault recovery capabilities, comprises a plurality of optical fibers and a plurality of nodes that exchange optical signals over the optical fibers. The plurality of nodes are arranged according to a prescribed topology. Each of the nodes comprises an optical switch that is connected to the optical fibers and is configured to switch the optical signals among the optical fibers. The optical switch includes a plurality of unit optical switches and a common driving mechanism that is configured to perform simultaneously switching operation of the unit optical switches to alter the switching state of the optical switch. The node also includes a plurality of monitoring devices that are coupled to the optical fibers. The monitoring devices are configured to monitor the optical signals that are transmitted over the optical fibers and to output selectively monitoring signals that indicate one or more faults in the optical fibers. Further, each of the nodes includes a control device that is coupled to the monitoring devices and is configured to output a control signal to the optical switches to effect an optical protection scheme by selectively changing switching states of the optical switch in response to the monitoring signals. Under this arrangement, a high reliable optical communications network is achieved, in part by decreasing the probability of switch malfunction by effecting a protection function using a minimal number of switch operations.
In yet another aspect of the invention, an optical switching device comprises a first optical switch and a second optical switch that is coupled with the first optical switch to form a common switching fabric. Each of the optical switches comprises two unit optical switches and a common driving mechanism that is configured to simultaneously operate the two unit optical switches. This arrangement permits the use of a minimal number of optical switches that are utilized in an optical communications network with fault recovery capabilities.
Other features and advantages of the present invention will become readily apparent from the following description taken in conjunction with the accompanying drawings.